In late years much attention has been focused on novel power generating techniques which are superior in energy efficiency or environmental friendliness. In particular, solid polymer fuel cells using solid polymer electrolyte membranes are characterized as exhibiting high energy density and being started and stopped more easily than fuel cells of other systems due to their lower operating temperature. Therefore, they are on development as generators for electric motorcars, dispersed power generation and the like. In addition, development of direct methanol fuel cells which use solid polymer electrolyte membranes and into which methanol is supplied directly as fuel are underway for applications such as electric sources of portable devices. Proton-conducting ion exchange resin films are usually used for solid polymer electrolyte membranes. Solid polymer electrolyte membranes are required to have characteristics such as fuel permeation inhibitability and mechanical strength preventing permeation such as hydrogen of fuel is necessary for a solid polymer electrolyte membrane as well as proton conductivity. As such a solid polymer electrolyte membrane, for example, per fluorocarbon sulfonic acid polymer membranes in which sulfonic acid groups are introduced, typified by Nafion (commercial name) manufactured by Du Pont, U.S.A., are known.
In order to enhance output power and efficiency of solid polymer fuel cells, it is effective to reduce the ion conduction resistance of solid polymer electrolyte membranes. One of the measures for it is to reduce the thickness of membranes. For solid polymer electrolyte membranes typified by Nafion, reduction of membrane thickness has been attempted. However, those membranes have problems in that when the thickness is reduced, the mechanical strength decreases and, as a result, when a solid polymer electrolyte membrane and an electrode are bonded together by hot pressing, the membrane easily ruptures or, due to the change of dimensions of the membrane, the electrode bonded to the solid polymer electrolyte membrane peels off, resulting in lowering of electric power generation characteristics. Moreover, they have problems in that the reduction of the thickness results in lowering of the fuel permeation inhibitability, which reduces electromotive force or efficiency for fuel utilization.
Moreover, solid polymer electrolyte membranes can be applied for a wide variety of applications including, in addition to an application as an ion exchange resin membrane of fuel cells shown above, applications in the field of electrochemistry such as electrolytic applications e.g. alkali electrolysis and production of hydrogen from water and electrolyte applications in various types of cells e.g. lithium cells and nickel hydrogen cells; mechanical functional material applications such as micro actuators and artificial muscles, applications for functional materials for recognizing/responding to ions, molecules and the like; and applications for functional materials for separation/purification. It is conceivable that, in each application, new, superior functions will be offered by increasing the strength of solid polymer electrolyte membranes or reducing the thickness thereof. As a method for improving the mechanical strength of a solid polymer electrolyte membrane and controlling its dimensional change, composite solid polymer electrolyte membranes resulting from combining solid polymer electrolyte membranes with various types of reinforcing materials have been proposed. Patent Document 1 discloses a composite solid polymer electrolyte membrane prepared by allowing a per fluorocarbon sulfonic acid polymer, which is an ion exchange resin, to soak into voids of a drawn porous polytetrafluoroethylene membrane and uniting them. However, these composite solid polymer electrolyte membranes have problems in that the reinforcing material is easily softened by the heat generated during electric power generation because it is made of polytetrafluoroethylene and, therefore, the membrane tends to change in dimension due to creep, that when the reinforcing material is impregnated with per fluorocarbon sulfonic acid polymer solution and then dried, the capacity of the voids in the reinforcing material causes almost no change and, therefore, the per fluorocarbon sulfonic acid polymer solidified in the voids of the reinforcing material tends to be unevenly distributed, that to fill the voids completely with the polymer requires a complicated process such as repeating twice or more the process of impregnation with the ion exchange resin solution and drying, and that it is difficult to obtain a membrane with a superior fuel permeation inhibitability because voids tend to remain. Patent Document 2 discloses a composite solid polymer electrolyte membrane in which fibrillated polytetrafluoroethylene as a reinforcing material is dispersed in a membrane made of per fluorocarbon sulfonic acid polymer. However, the composite solid polymer electrolyte membrane has a problem in that delamination of the electrodes occurs because the membrane can not exhibit sufficient mechanical strength and therefore the deformation of the membrane can not be controlled because the membrane has a structure where the reinforcing material is discontinuous. Moreover, Patent Document 3 discloses an electrolyte whose creep elongation at high temperatures is reduced through improvement in heat resistance achieved by crosslinking side chains multifunctionalized. However, the creep elongation of the electrolyte in Patent Document 3 is defined based on a deformation occurring during a very short time as short as four minutes and there is no discussion about the effect of moisture. When it is exposed to a high temperature, humidified atmosphere for a long period of time, lowering of heat resistance caused by decomposition of the side chain crosslinking structure introduced or deformation caused by relaxation of a secondary structure of a main chain are unavoidable. Therefore, a measure such as that described in Patent Document 3 has a problem in that no electrolyte can be achieved which exhibits a small creep deformation under a load applied for a long period of time under a high temperature, humidified atmosphere important for practical use of solid polymer fuel cells.
Polybenzazole polymers such as polybenzooxazole (PBO) and polybenzimidazole (PBI) are expected to be suitable as a reinforcing material of solid polymer electrolyte membranes because they are superior as having a high heat resistance, a high strength and a high elastic modulus Patent Document 4 discloses solid polymer electrolyte membranes in which a PBO porous membrane is combined with various types of ion exchange resin. However, it has problems in that on both surfaces of a PBO porous membrane obtained by a method including solidifying, directly in a water bath, a PBO solution film formed from a dope which exhibits mesomorphism such as that disclosed in that document, dense layers having less apertures are formed; when the membrane is combined with ion exchange resin, an ion exchange resin solution is difficult to be soaked into the membrane, resulting in a low content of the ion exchange resin in a composite membrane, and characteristics such as ionic conductivity inherent to the ion exchange resin are greatly deteriorated. Moreover, the composite ion exchange membrane disclosed in this document is not particularly restricted with respect to the formation or thickness of surface ion exchange resin layers. However, the presence and thickness of surface layers in a composite ion exchange membrane have an effect on adhesion between an ion exchange resin serving as a binder and an ion exchange resin forming solid polymer electrolyte membranes and it is important to optimally control them.
Patent Document 5 discloses a method for manufacturing a polymer film for fuel cells, in the film an acid being trapped in voids of a PBI porous membrane. However, a film trapping a free acid therein obtained by a method such as that described in this document has problems in that its ionic conductivity in a low temperature range such as that up to 100° C. is lower than that of ion exchange resin membranes such as the aforementioned Nafion and that the acid tends to exude. Moreover, Patent Document 6 discloses a method for obtaining a polybenzazole film by forming a film from an optically anisotropic polybenzazole polymer solution and solidifying the film through a process of rendering isotropic. However, a polybenzazole film obtained by a method such as that disclosed in this document is a transparent and highly dense film, which is not suitable for the purpose of converting it to an ion exchange membrane by impregnating it with ion exchange resin.
[Patent Document 1]
Japanese Patent Laying-Open No. 8-162132
[Patent Document 2]
Japanese Patent Laying-Open No. 2001-35508
[Patent Document 3]
Japanese Patent Laying-Open No. 2002-324559
[Patent Document 4]
Pamphlet of International Publication No. WO00/22684
[Patent Document 5]
Pamphlet of International Publication No. WO98/14505
[Patent Document 6]
Japanese Patent Laying-Open No. 2000-273214
The present invention provides a composite ion exchange membrane which has high mechanical strength and is suitable for use as a solid polymer electrolyte membrane excellent in ionic conductivity and a method for its production and, furthermore, an electrolyte membrane-electrode assembly having good adhesion between the electrolyte membrane and the electrode assembly.